Integrated electrical motors and pumps are known. These systems function to apply torque to a pump to supply power for driving the pump. See U.S. Pat. Nos. 2,519,580, 3,132,595, 3,295,457, 3,514,221, 3,672,793, 4,529,362 and 4,606,707. None of the aforementioned patents discloses a system used for starting engines.
Aircraft engines have been started by using an alternator which is used to generate aircraft power as a motor. For example, see U.S. Pat. No. 4,473,752. These systems operate the alternator as a synchronous motor or a brushless DC motor. These systems do not couple heat generated during starting by the windings of the motor-generator set to hydraulic fluid within a case containing a hydraulic pump-motor combination which is part of a conventional constant speed drive transmission in an aircraft power generation system. Use of the generator as a synchronous motor to start an aircraft engine requires a mechanism for accelerating the rotor of the generator close to synchronous speed such as an induction motor or brushless DC motor. If the starting torque produced by the induction or brushless DC motor to accelerate the generator to synchronous speed is applied through a constant speed drive transmission having a pair of conventional hydraulic units disposed in a case full of hydraulic fluid as described below, cold weather conditions can cause the hydraulic fluid to have high viscosity which prevents the development of sufficient starting torque. An induction or brushless DC motor having sufficient starting torque to accelerate a generator close to synchronous speed to use the generator as a synchronous motor to start an aircraft engine with the starting torque applied through a constant speed drive transmission having a hydraulic pump and motor immersed a full case of hydraulic fluid must be designed to produce sufficient torque to accelerate the hydraulic pump and motor under cold weather conditions at which the hydraulic fluid has a high viscosity prior to connecting the generator to the engine to be started by a torque link.
FIG. 1 illustrates a block diagram of a prior art starting system for an aircraft engine utilizing a constant speed drive. This system relates generally to the type of system disclosed in U.S. Pat. No. 4,748,337 which is assigned to the assignee of the present invention. Patent application Ser. No. 112,701, filed Nov. 30, 1987, which is assigned to the assignee of the present invention, discloses a system for starting an engine with a hybrid permanent magnet induction motor which may be part of a brushless generator. The starting system 10 consists of a conventional constant speed drive (CSD) or an integrated drive generator (IDG) sold by the assignee of the present invention which includes a hydraulic pump and motor 12 combination which is disposed within a case 14 full of hydraulic fluid 16. As is well known, a CSD and an IDG function to provide a constant shaft speed applied to a three phase alternator from a variable speed shaft input from a power takeoff from an aircraft propulsion engine. The difference between an IDG and a CSD is that the IDG combines the generator and a constant speed drive transmission providing constant frequency electric power integrated in a single unit, while in a CSD the constant speed drive transmission is separated from the generator. The hydraulic pump and motor combination 12 is a conventional configuration. An aircraft engine (not illustrated) is rotationally coupled to the remainder of the CSD or IDG 18 may include a differential. The remainder of the CSD or IDG 18 is not illustrated for the reason that it is conventional and is not important in understanding the present invention. A driven shaft 20 is connected to the hydraulic pump and motor combination 12. The hydraulic pump and motor 12 combination has an output shaft 22 which is connected to a motor-generator set 24 which is a three phase alternator. Induction motor 26 is connected to the motor-generator set 24 to accelerate the motor-generator setup close to synchronous speed at which point the hydraulic pump and motor combination 12 is operated as a controlled variable torque link during which starting torque is progressively increased in magnitude from zero by the hydraulic pump and motor combination 12, and applied to the remainder of the CSD or IDG 18, which is connected to the aircraft engine to be started. The induction motor 26 is not intimately thermally coupled to the hydraulic fluid 16 within the case which prevents significant transfer of heat from the windings of the motor to the hydraulic fluid within the case during starting.
The full case 14 has a disadvantage that in cold weather conditions the viscosity of the hydraulic fluid 16 increases substantially with a change in viscosity occurring between 60.degree. F. and -40.degree. F. being from 200 centistokes to 13,000 centistokes. At cold temperatures, the viscosity of the fluid is so great that the starting torque of the induction motor 26 must be extremely high to cause initial rotation of the hydraulic pump and motor combination 12 to accelerate the motor-generator set close to synchronous speed so that motor/generator set 24 may be locked into synchronism and starting torque may be applied to the engine to be started. Furthermore, because of the nature of the full case 14, initial rotation of the aircraft engine produced by the remote induction motor 26 does not substantially heat the hydraulic fluid 16 within the case which makes it extremely difficult for the induction motor to rapidly accelerate the hydraulic pump and motor combination 12 close to synchronous velocity at which motor-generator set 24 may be operated as a synchronous motor to initiate rotation of the aircraft engine during starting through a variable torque link. In order to insure operation of the starting system to start an aircraft engine at low temperatures, it is necessary to size the induction motor 26 to have sufficient starting torque to initially rotate the hydraulic pump and motor combination 12 of the constant speed drive at temperature conditions which cause the aforementioned high viscosity of the hydraulic fluid 16 within the case 14. The sizing of the induction motor 26 in this manner represents a weight penalty.
The case 14 typically contains a charge pump 28, filter 30 and charge relief valve 32 which function as follows. Hydraulic fluid 16 is taken in through intake 34 by charge pump 28 through filter 30 to the hydraulic unit in the hydraulic pump and motor combination 12 which is functioning as a pump. Any over pressure of the hydraulic fluid in the system is relieved by charge relief valve 32. As a result of the volume of the case, exposure of the case to cold weather conditions creates a volume of chilled hydraulic fluid of high viscosity which will place immediate drag on the hydraulic pump and motor combination 12 and which must be warmed to reduce the aforementioned drag.
During normal operation the hydraulic fluid circuit external of the case functions as follows. Hydraulic fluid flows from output port 38 through check valve 40 into external sump 42. Hydraulic fluid 43 within the external sump 42 is pumped by scavenge pump 44 through scavenge filter 46 to heat exchanger 48. The heat exchanger functions to cool the hydraulic fluid which is then discharged into the case through check valve 50 at input port 52.